The present invention relates to a vessel of the OBO or bulk carrier type for transportation of bulk cargo, comprising transverse bulkheads.
An OBO vessel (ore/bulk/oil) is a vessel which can carry cargo in both liquid and dry form, and a typical use is alternation between transportation of crude oil and coal. A vessel of this type is also known as a PROBO (product/ore/bulk/oil) carrier or a combination carrier. A bulk carrier is a vessel which can carry a variety of bulk commodities mostly in dry form. OBO""s and bulk carriers are single deck vessels in which the deck is important for the structural strength of the vessel. The vessel is divided into several cargo holds by transverse bulkheads with access from above provided by hatches in the deck. Each hatch opening is normally covered by a two-piece hatch cover, each of which piece is movable in the transverse direction of the vessel, and is placed on the deck between the hatch coaming and the side of the vessel when the hatch is open.
The width of the hatches, i.e. the extent of the hatches in the transverse direction of the vessel, is much smaller than the beam of the vessel, normally 50 percent of the beam of the vessel. The reason for this is two-fold: Firstly, larger hatches would not render sufficient space on deck for placing the hatch covers between the hatch coaming and the side of the vessel, and secondly, larger hatches would impair the structural strength of the vessel by rendering insufficient structural steel in the deck.
Loading of bulk cargo is normally carried out by conveyor belt/shiploaders or grabs which drop the cargo vertically into the hold. The relatively small hatch openings compared to the beam of the vessel means that the loading equipment""s access to the outer parts of the hold is hindered by the deck, which forms an xe2x80x9coverhangxe2x80x9d above this part of the hold, normally resulting in a need for extra trimming.
Discharging of bulk cargo from a hold is normally carried out by discharging equipment temporarily located above the hatch, comprising a gantry crane with a large grab which is lowered into the bulk cargo, and which to a limited extent is moveable in the transverse direction of the hold. Again the relatively small hatch opening compared to the beam of the vessel is a limitation, as the xe2x80x9coverhangxe2x80x9d created by the deck hinders the grab""s access to the outer parts of the hold. To get better access to the outer parts of the hold, the grab is often forced in the transverse direction of the hatch, which may cause damage to the hatch coamings. As a considerable amount of cargo is unreachable by the grab, a caterpillar is lowered into the hold to move the bulk cargo from the outer parts of the hold into the area which is accessible to the grab/discharging equipment, which is a time-consuming and costly operation.
When transporting dry cargo the hatches are closed, the hatch covers being tightened and secured to the hatch coamings.
In tanker mode, the hatch covers of the OBO vessel are in closed position, tightened and secured to the hatch coamings, both during loading, unloading and transport. The liquid cargo is loaded through the main cargo lines, via the drop lines, and discharged by the cargo pumps via the main cargo lines. The upper space of the cargo hold is filled with inert gas under pressure. Overpressure from vaporisation of the liquid cargo is controlled by a pressure vacuum valve situated in the highest point of the cargo hold. To prevent leakage through the hatch openings, leakage seals are provided between the hatch covers and the coamings.
However, the vessel is not stiff, and the load distribution, wave movements and also uneven thermal expansion due to uneven exposure to sunlight, will cause the deck and hatch coamings to bend and deflect slightly. Despite the tightening and sealing of the hatch covers, leakage therefore still occur, particularly where the two-piece hatch covers are joined above each hold. The leakage problem is thus another problem related to OBO""s and bulk carriers according to prior art.
The leakage problem is also related to the material of the seals, which are normally made of a chemically resistant elastomer, such as rubber. In addition to being subjected to compression, the seals are exposed to the natural properties of the cargoes carried, such as oil or naphtha, as well as the environmental exposure from sunlight, extreme temperatures, seawater, and ice. So far an elastomer which is able to withstand all these exposures for a longer period, is unknown. A relatively frequent exchange of seals is therefore a further problem related to present OBO""s and bulk carriers.
The biggest problem related to OBO""s and bulk carriers, is the movement of the cargo in the hold, so-called sloshing, which may cause transverse displacement of the centre of gravity, tilting and instability of the vessel.
Tankers may have piping located in the cargo tanks. In bulk carriers, however no piping can be located in the holds, as this would prevent access for the loading and discharging equipment and complicate the cleaning of the vessel. In OBO vessels piping is therefore located on the deck where it is exposed to the environment and the cargoes carried. This exposure and sometimes extreme corrosion, caused by certain types of dry cargo mixed with seawater, and also the mechanical damages caused by various equipment used during the operation of the vessel, gives raise to the need for adequate maintenance, which again means increased operational costs for the vessel.
U.S. Pat. No. 3,698,347 discloses a vessel of the OBO type in which the holds are serviced by a pair of transversely disposed hatches and a central longitudinal girder forms a ventilating trunk extending lengthwise of the vessel and a support for the adjacent hatch coamings of the transversely disposed hatches. The hatch covers are raised and lowered by jacks and one of the pair of hatch covers of the transversely disposed hatches is movable transversely of the vessel to underneath the other of the pair of hatch covers, and both of the pairs of hatch covers are moveable xe2x80x9cpiggybackxe2x80x9d transversely of the vessel. This OBO design thereby enables a better access to the cargo hold than the OBO discussed above. However, the problem related to the sloshing during carriage of oil or other liquid cargo is not solved. The leakage problem in tanker mode might be reduced by the central, longitudinal girder, however, as the girder will be subjected to bending and deflection, as discussed for the deck above, the leakage problem is therefore not solved.
The main object of the invention is to provide a vessel of the OBO or bulk carrier type which has a much better stability compared to known designs. There is also an object to provide a vessel of the OBO or bulk carrier type which is stronger built and therefore better suited for having hatch covers fitted on the deck, and also to carry heavier cargoes such as ore compared to known vessels. A further object is to provide a vessel of the OBO or bulk carrier type with an xe2x80x9copen hatchxe2x80x9d concept which is self-trimming and self-draining, and in which the access of the loading and discharging equipment is significantly improved.
A further object is to provide an OBO or bulk carrier in which the problems related to leaking hatches shall be eliminated or at least significantly reduced. It is also an object to provide an OBO or bulk carrier in which a hold can be loaded or discharged completely without significantly influencing the vessel""s list. A further object is to solve the problem related to piping on deck being exposed to the environment and damages during cargo operations.
These objects are achieved by a vessel of the OBO or bulk carrier type according to the invention which comprises transverse bulkheads and at least one longitudinal centreline bulkhead intersecting the transverse bulkheads. The transverse bulkheads and the longitudinal centreline bulkhead form longitudinal cargo holds. The longitudinal form of the cargo holds and hatches facilitate unloading by the grab of a gantry crane located above the hatch, and is also advantageous with respect to strength and stiffness of the vessel.
The centreline bulkhead strengthens the deck, and enabled by this, the cargo holds are provided with large hatch openings and single piece hatch covers. The width of the hatch openings of the vessel according to the invention is essentially the same as the width or beam of the tank-tops, i.e. the bottom of the cargo holds, thus creating an xe2x80x9copen hatchxe2x80x9d which improves the trimming of bulk cargoes significantly. It also gives the discharging equipment, such as grabs a better access to the holds during discharging of bulk cargoes and thereby reduces the risk of stevedore damages.
This is in contrast to large OBO""s and bulk carriers according to prior art which have hatch openings traverse the length of the vessel, the width of these openings being rather narrow compared to the width of the tank-tops, creating a considerable xe2x80x9coverhangxe2x80x9d, as mentioned above.
The centreline bulkhead of the vessel according to the invention extend essentially through the length of the ship. It should, however, be understood that the centreline bulkhead may be dispensed with in areas of the vessel with no or small cargo holds, e.g. in the engine room and in the bow or stern of the vessel.
Further the OBO or bulk carrier vessel according to the invention preferably has vertical corrugations in the longitudinal centre-line bulkhead and in the transverse bulkheads of each hold. Such corrugations are known per se, however, the corrugated bulkheads of existing OBO""s and bulk carriers are differently shaped, i.e. they are mostly shaped in a 90 degree pattern including the bottom plate of the corrugations (where the hopper starts) which traps the cargo inside the corrugations during discharge of bulk cargoes such as coal. By the invention the cargo will not be trapped, as all corrugations, including the bottom plate, are sloping at least about 30 degrees which will save considerable time in discharging and cleaning of the vessel. The holds in a vessel according to the invention will thus have excellent self-trimming and self-draining properties.